1. Field of the Invention
The present invention relates to a numerical controller for machine tools, and more particularly to a numerical controller for machine tools that controls a speed in the tangential direction in arc operation by using an arbitrarily set or commended frequency or angular speed.
2. Description of the Related Art
In a numerical controller for machine tools, speed control in an arc-shaped machining path is performed so that the acceleration caused by changes in the movement direction of an axis is equal to or less than an arbitrarily set or specified acceleration. Even when the same acceleration is set, however, the frequency to be commanded for a servo system increases as the arc radius becomes smaller or the commanded speed becomes larger. As a result, the servo system may become unstable when the response cycle of position control is exceeded. A general measure against this case is to stop the operation of the control axis by an alarm that is issued by the servo or numerical control. If the axial operation stops during machining, however, the workpiece may be damaged or it takes long time until the machining is resumed.
An allowable acceleration is arbitrarily preset or specified within the range of machining that is actually performed, so that speed control is performed not to saturate the specified speed value or specified torque command value. In speed control only by the allowable acceleration, however, a problem arises that the feedrate cannot be clamped sufficiently during machining of a minute arc or an attempt to sufficiently clamp the feedrate increases machining time considerably.
Japanese Patent Application Laid-Open No. 2003-334740 discloses a technique in which a speed command for each time is generated on the basis of a specified route and specified speed, the frequency band components corresponding to machine vibration included in the generated speed command for each time are calculated to obtain the frequency components corresponding to the machine vibration, thereby obtaining an allowable speed that allows the obtained frequency components to be equal to or less than a reference value.
Japanese Patent Application Laid-Open No. 63-303402 discloses a technique for correcting a speed command for an arc path using the target value of an angular speed value about the arc center. This technique is speed control that corrects an arc route and reduces the path difference between the specified path and the response path by feed forward control.
Japanese Patent Application Laid-Open No. 2003-334740 and Japanese Patent Application Laid-Open No. 63-303402 do not relate to speed clamping for machining shapes by interpolation of an arc with a small radius.